Evaluating Motivation and Reward Mechanisms and Brain Substrates in Adults With Obesity

The EEfRT task is a behavioural paradigm that is used to investigate the subject's effort-based decision-making. Subjects are presented with a series of repeated trials wherein they must choose between performing a hard task or an easy task in order to receive monetary compensation (subject's will receive greater compensation for completing the hard task and less compensation for completing the easy task). By presenting the subject with different probability levels for reward receipt, the objective is to examine the extent to which the relationship between motivation (i.e., anhedonia) and effort-based decision-making is modulated by perceived reward magnitude.

The MID is a behavioural paradigm that assesses the basis of anticipation and consumption of reward and punishment. Each run of the MID task consists of approximately 72 trials. The types of cues presented include: circles (indicating the potential to win money i.e., gain cue), squares (indicating the potential to lose money i.e., loss cue), and a triangle (indicating no money will be won or lost i.e., neutral cue). A cue is presented for 500 ms, followed by a fixation cross (2,000 to 2,500 ms) and then the target square (160 to 260 ms), during which the participant is instructed to press a button as quickly as possible to win or avoid losing money. A feedback screen (1,650 ms), in which the top number indicates the amount of money won or lost during that trial, and the bottom number indicates the participant's total amount, is presented at the end of each trial.

The PRT is a behavioural paradigm that measures response bias and sensitivity to reward under variable conditions. In this task, a signal detection approach is used, whereby subjects must discriminate between two non-ambiguous stimuli displayed rapidly on a computer screen in order to receive a monetary reward. Unbeknownst to the subjects, correct identification of one stimulus is reinforced three times more frequently than the other stimulus. Under these experimental circumstances, healthy subjects reliably develop a response bias for the stimulus that is reinforced more frequently, regardless of which stimulus was actually presented. Thus, reward responsiveness assessed in this task reflects the rapid shaping of future behavioral choices based on prior reinforcement experiences.

The secondary outcome will be the association between the EEfRT and function reciprocity of the reward circuit/cognitive control network as measured by MRI/DTI.

Subjects will be scanned using a 3.0-Tesla Signa HDx scanner with an 8-channel phased-array receiver coil consisting of a structural and functional neuroimaging, comprising:

• Whole-brain 3-D T1-weighted Inversion-Recovery prepared Fast Spoiled Gradient-Echo anatomical scan
• Whole-brain, T2*-weighted BOLD echo planar imaging during awake resting state
• Three runs of whole-brain, T2*-weighted BOLD EPI series during task-based reward paradigm.

Graph theoretical analysis will be used to assess functional connectivity of the default mode network, cognitive control network, affect network and reward network.

Associations between obesity and alterations in functional connectivity will be the outcome of interest.